The present invention relates to aqueous, heat-curable amino resin mixtures based on etherified amino resins and formaldehyde-binding auxiliaries and to their use for impregnating papers and cardboard.
Surfaces and narrow faces of woodbase materials are coated using coated foils (finished foils) and, respectively, impregnated cardboard (Kunststoff-Handbuch Vol. 10 Duroplaste [Thermosets], Hanser-Verlag, 2nd Ed. 1988, p. 462 f., pp. 477 to 479). For the production of finished foils (to coat surfaces) and finished edgings (to coat narrow faces, of boards in particular), absorbent papers are impregnated with solutions or dispersions of (thermosetting) amino resins such as urea-formaldehyde and/or melamine-formaldehyde resins, for example, alone or in combination with dispersions of thermoplastics such as acrylic dispersions or styrene-acrylic dispersions, dried in a stream of hot air and simultaneously cured, and then coated.
In order to ensure adequate penetration of the impregnation liquors into the paper or cardboard, the resin solutions must be processed from aqueous or alcohol dilution. Owing to the high level of cellulose swelling in aqueous systems, the foils and edgings produced from high aqueous dilution are brittle, exhibit a high level of water absorption, and even in the coated state have a surface whose visual appeal is low. The procedure described in DE-A 23 09 334, comprising impregnating liquors diluted with C1 to C4 alcohols, does give foils and edgings having good performance properties but requires complex measures for reprocessing the waste gas. The route to a solution that is described in DE-A 44 39 156, modifying melamine resins with guanamines, makes it possible to carry out impregnation from purely aqueous impregnating liquors. A disadvantage, however, is the significantly higher cost of the amino resin, resulting from the use of the guanamines.
The formaldehyde emissions of finished foils and edgings after manufacturing are determined following storage under standard climatic conditions (23xc2x0 C., 50% relative atmospheric humidity) in accordance with the standard EN 717-2 (FESYP method, gas analysis). Foils and edgings with values of less than 3.6 mg/(hxc2x7m2) by the FESYP method meet the limit of the standard (xe2x80x9cE 1xe2x80x9d). The rates of emission found remain more or less constant even after several weeks of storage under standard climatic conditions. Formaldehyde emissions observed on the films and edgings arise due to the use of urea-formaldehyde and/or melamine-formaldehyde resins in the impregnating liquors for impregnating the paper or cardboard sheets and/or in the coating formulations for coating the films and edgings. By using particularly low-formaldehyde urea-formaldehyde and/or melamine-formaldehyde resins (with low formaldehyde clearage) it is possible to reduce the formaldehyde emissions as measured by the FESYP method (standard climatic conditions) to values around 2 mg/(hxc2x7m2).
It has surprisingly now been found that when edgings produced in accordance with the prior art and originally (before the commencement of storage) satisfying the classification E1 (xe2x80x9cE1 edgingsxe2x80x9d), with initial formaldehyde emission values of from 1.0 to 3.5 mg/(hxc2x7m2), are stored under nonstandardized climatic conditions, at customary summer temperatures and atmospheric humidities, the formaldehyde emissions rise in the course of a few weeks to values of in some cases much higher than 3.5 mg/(hxc2x7m2), and so the edgings no longer meet the E1 criterion. This unexpectedly high increase in the formaldehyde emissions was confirmed by storage under defined conditions in a tropical climate (35xc2x0 C., 90% relative atmospheric humidity), with measurement being carried out only after 3-day reconditioning under standard climatic conditions following storage under the tropical climatic conditions.
From the prior art it is known that the amount of free formaldehyde and also the formaldehyde emissions may be reduced by adding formaldehyde scavengers such as urea and urea derivatives, for example. For instance, according to DE-A 38 37 965, finished foils and edgings with formaldehyde emissions that are negligible as determined in accordance with DIN 52368 may be produced by adding urea to the melamine-formaldehyde condensation product. Regarding the behavior during and after storage under tropical climatic conditions, however, no information is given. According to DE-A 34 03 136, mixtures of organic hydroxy compounds and an amide are suitable for use as formaldehyde-binding agents in boards made from wood cellulose materials. The use of these mixtures as formaldehyde scavengers in finished foils and edgings is not described. The addition of the mixtures described in DE-A 34 03 136 to amino resins that are used to produce finished foils and edgings leads to a marked deterioration in the flexibility of the finished foils and edgings produced with them. The use of formaldehyde scavengers known from the literature, such as urea, ethyleneurea and propyleneurea, resulted in finished foils and edgings which met the E1 criterion under standard climatic conditions but which markedly exceed the E1 limit of 3.5 mg/(hxc2x7m2) under tropical climatic conditions.
It is therefore an object of the present invention to develop amino resin mixtures for producing films and edgings, which can be used to produce, relative to the prior art, finished foils and furniture edgings with significantly reduced formaldehyde emission when stored under tropical climatic conditions, while retaining the required performance properties.
The invention provides aqueous amino resin compositions comprising amino resins A, formaldehyde-binding additives (auxiliaries) B, which may comprise hydroxyl group-containing polyurethanes B13, and, if desired, acrylic resins C in the form of aqueous dispersions, and water.